In Situ STM and AFM Study of Protoporphyrin and Iron(III) and Zinc(II) Protoporphyrins Adsorbed on Graphite in Aqueous Solutions

Iron(III) protoporphyrin(IX), zinc(II) protoporphyrin(IX), and protoporphyrin(IX) have been studied on the graphite basal plane in aqueous solutions with both scanning tunneling (STM) and atomic force (AFM) microscopies. Real-time images directly show that the molecules adsorb onto the electrode and condense into monolayer films starting from small islands. In the monolayer films, all three molecules lie flat on the surface and form identical two-dimensional lattices with a = 13.4 ± 0.2 Å, b = 12.2 ± 02 Å, and y = 68 ± 2°. The corresponding molecular packing density is 1.1 x 10-¹⁰ mol/cm², which is in excellent agreement with the value determined from cyclic voltammograms. Although the three molecules are nearly identical in their geometrical structures, their internal structures revealed by STM are significantly different. After the monolayer is completed, iron(III) protoporphyrin(IX) forms aggregates with a thickness of ∼8.5 Å, while zinc(II) protoporphyrin(IX) and protoporphyrin)IX) do not.